Method for drying



July 15, 1941. F, R. BIYCHOWSKY METHOD FOR DRYiNG Filed July 26, 1959 Mw o M T/ N5 E. v@ 5 C m ATTORNEY Patented July 15, 1941 UNITED STATESPATENT OFFICE METHOD FOR DRYING Application July 26, 1939, Serial No.286,536

2 Claims.

This invention relates to methods of drying materials. More particularlyit concerns an improved method for removing moisture from materials bybringing into contact therewith a relatively dry atmosphere ofcontrolled temperature and humidity, this atmosphere being subsequentlyconditioned for re-use by contact with a hygroscopic liquid.

One of the objects of the invention is to provide a highly efficientmethod of drying moist materials. Another object is to define a cyclicprocess for conducting drying operations under conditions which aresubstantially adiabatic and largely independent of the surroundingatmosphere. 1

With these and other objects in view, the present invention contemplatespassing a stream of air of predetermined temperature and relativehumidity in a closed system into contact with materials to be dried anddirectly thereafter into contact with a hygroscopic solution ofcontrolled temperature and concentration, whereby the air is brought tothe proper condition for recirculation in the process without having tobe rejected or replaced in part with fresh air. This continued re-use ofthe same air permits large heat savings in the process.

The invention may best be understood with reference to the accompanyingdrawing which illustrates diagrammatically a preferred embodiment of theinvention.

In the apparatus illustrated, drying is conducted in a chamber I withshelves 2 for holding the, material to be dried. Air is circulatedthrough the drier in a closed circuit comprising a motor-drivenair-circulating fan 3, an air delivery duct 4 connecting the fan to thechamber l, and an outlet duct 5 connected through a damper 6 to theair-conditioning chamber 1 and through a damper 8 to the by-pass 9, theopposite end of both the chamber 1 and the by-pass 9 being connected toa duct Ill and by it to the inlet side of the fan 3. The conditioningchamber 'I is part of a circulatory system for the hygroscopic liquid,the system also comprisinga reservoir II, a liquid sump l2, and a pumpl3 connecting the sump through lines l4 and I5, and. a valve Hi to thereservoir ll. circulating system comprising the chambers l and I andtheir associated ducts, and the liquid circulating system comprisingpump l3 and connecting piping, are preferably provided with insulatinglagging (not shown) to prevent transmission of heat into or out of thesystem.

Inasmuch as the hygroscopic solution in the liquid circulating systembecomes diluted during operation, a small portion of the solution iswithdrawn from the line H through a valve I1 and is conveyed through aline [8 and a heatexchanger jacket I9 into an evaporator or regenerator20 which is fired by a gas burner 2| controlled by a gas valve 22.trated liquor from the bottom of the regenerator is then returned to theliquid circulating system through a heat-exchanger coil 23, or a by-pass24 controlled by a valve 25, and thence through a return line 26 leadingto the sump I2. This line 26 is provided with a small heat exchanger 21to which cooling water may be admitted as needed through a valve 28.

The concentration of the hygroscopic solution in the apparatus may becontrolled by any desired means, suitably by varying the rate ofevaporation .in the regenerator 20, e. g. by adjusting the gas-flow tothe burner 2|. This control may be efiected automatically by means of adensity indicator 29 receiving hygroscopic liquid through a by-pass 30,the density-responsive plummet 3| of the device being connected to theburner valve 22 (as shown by the solid line) in such manner that the gasflow to the burner is varied in accordance with the indications of theplummet 3|. The temperature and humidity of the air circulated throughthe driers are regulated by a thermostat 32 and a wet-bulb thermometer33' located within the drying chamber l, the thermostat 32 being adaptedto actuate the valve l6 controlling the rate of circulation of thehygroscopic liquid, and the Wet-bulb thermostat 33 being operativelyconnected to valve 24 controlling the gby-pass 23 (as shown by the fullline), and simultaneously or alternatively to the valve 28 controllingthe cooler 21 (as shown by the broken line).

In operating the process according to the invention, the materials to bedried are placed upon the shelves 2 or otherwise conveyed into thedrying chamber, and the fan 3 is set in motion causing the air in thesystem to circulate through the chamber I in contact with the materialto be dried. In this chamber, since in 0p- Both the aireration thepartial pressure of water in the air is less than the vapor pressure ofthe moisture in the material being dried, this moisture will evaporateinto the air stream, such evaporation cooling the air and increasing itshumidity. Inasmuch as the chamber l is thermally insulated, thisevaporation and cooling occurs under essentially adiabatic conditions,and the air remains The reconcen- I at substantially constant totalheat, 1. e. at constant wet-bulb temperature.

The cooled moist air leaving the chamber I passes through the duct 5 anddamper 6 into the conditioning chamber 1, the by-pass damper 8 beingclosed in this mode of operating. In the chamber I the moisture-ladenair is brought into contact with a hygroscopic liquid circulated by thepump I3, either in the form of a shower or spray, or in distributed formupon a suitable medium such as cotton strings suspended from thereservoir H, as shown. In this air-liquid contact zone the hygroscopicliquid absorb part of, usually most of, the moisture from the air, suchabsorption being accompanied by a 1ibera-. tion of the heat ofcondensation and solution of the water vapor, this heat raising thetemperature of the hygroscopic liquid, which, in turn, transfers a partof this heat back to the air in contact therewith. The air, thus warmedand dried, is ready for recycling in the process through the duct ID tothe inlet of the fan 3. As operation is continued, the hygroscopicliquid being circulated into contact with moist air in the chamber 1continues to heat up, as explained, until it reaches and remains at sometemperature above that of the incoming air, at which tem perature theamount of heat transferred from the liquid to the air is exactly equalto the heat liberated in the dehumidification of air. Now, since thechamber '1 is thermally insulated and since, as will be explained, thehygroscopic liquid circulating through the reservoir ll, sump l2, pumpl3, and pipes l4 and I5 does not change in temperature appreciably andis maintained at constant composition, it is seen that the heating anddehumidification of the air occurring in the chamber 1 takes place underadiabatic conditions, i. e. the air remains at constant heat content andhence at a constant web-bulb temperature.

As hereinbefore explained, the hygroscopic liquid being circulatedthrough the chamber 'lis maintained at constant concentration bywithdrawing a small portion, e. g. 5-10 per cent of the total, andremoving water therefrom, as in the evaporator 20, which is controlledby the density-responsive plummet 3|. The reconcentrated liquor returnedfrom the evaporator through the pipe 26 should be at substantially thesame temperature as the diluted portion withdrawn through line l8 inorder to avoid adding or subtracting heat from the otherwise adiabaticcirculatory system for the hygroscopic liquid. Thisnecessary'temperature control may given off by the material being dried.If a condition of inequality prevails, the temperature and humidity ofthe air passing into the drying chamber I will not remain constant, butwill vary somewhat as the drying proceeds. In certain instances thisvariation is of no great consequence, but for drying numerous materials,especially heat-sensitive products, such as gelatine, it is highlydesirable that the drying air be supplied at constant conditions oftemperature and humidity.

In order to maintain a constant temperature and humidity of the dryingair, it is necessary to insure: (1) that the air-circulation systemremain essentially adiabatic, that is, at constant wet-bulb temperature,and (2) that the airreconditioning step in chamber 1 be carefullycontrolled. These requirements will now be considered in detail.

(1) The process of the invention, since it involves closed cycles ofboth the air and the hygroscopic liquid, would of necessity remainadiabatic if it were not for the possibility of heat loss or gain in theliquid drawn off to the rgenerator 20, for heat transfer by radiationbetween the room and the walls of the chambers l and I and connectingducts and piping, and for slight frictional effects. In the preferredoperation of the process, there is little heat loss or gain in theregenerator circuit since the reconcentrated liquor may, by means of theheatexchanger 23, be returned through the pipe 25 at substantially thesame temperature as the diluted liquor withdrawn through the pipe l8, ashereinbefore explained, However, although heat transfer to and from theroom is minimized by covering the apparatus with heat insulation, it canrarely be entirely eliminated. To this end, a control instrumentresponsive to variations in the total heat content of the system, suchas a wet-bulb thermometer 33, is placed in the drying chamber 1. Whenthe apparatus is to be run at a temperature above that of itssurroundings, so that the total heat content of the system tends todecrease because of radiation losses, the wetbulb thermometer 33 isconnected operatively to the valve 25 controlling the heat-exchangerby-pass 24, as illustrated; the valve 28 controlling the cooler 21 iskept closed. Then, when the total heat of the system falls, thethermometer 33 drops slightly in temperature, opening the valve 25, andallowing some of the reconcentrated liquor from the evaporator 20 toby-pass be efiected by any desired means, as by operat it has been foundthat unless means are employed to control the air-reconditioning step inthe chamber I there is no assurance that the amount of water absorbed bythe hygroscopic solution will precisely equal the amount of watertheheat-exchanger 23. Under this condition the. reconcentrated liquorreturns to the circulatory system through the pipe 25 at a highertemperature than the diluted liquor leaving throng ,jthe line It. Heatis accordingly added to the liquid circulatory system and is, of course,transferred to the air-circulatory system, since the two are in contactin the chamber 1, so that the total heat content of the entire apparatusrises until the desired value is reached, at which time the wet-bulbthermometer operates to close the valve 25. On the other hand, when thedrying apparatus is to be run at a temperature lower than that of itssurroundings so that the total heat of the system tends to increase byradiation into the chambers l and 1, the wet-bulb thermometer isconnected operatively to the valve 23 controlling the cooler 21, and theby-pass valve 25 is closed. Thus, when the total heat of the systemrises, the thermometer 33 operates to open the valve 28, cooling thereconcentrated liquor returning to the system in the pipe 25 to atemperature below that of the .diluted liquor leaving the pipe I3. Heatis extracted from the entire system until the desired heat content isattained, when the thermometer 33 closes the valve 28.

In an alternative structure of the apparatus, not illustrated, theheat-exchanger 23 may be omitted, in which case the small heat-exchanger21 is connected ,so as to be either a heater or cooler in response tothe demands of the wetbulb thermometer 33. In fact, in the broadestsense of the invention, the wet-bulb thermometer 33 may be used tocontrol make-up heating or cooling means located anywhere within the airor hygroscopic liquid circulating systems, since these systems areinter-communicating and are otherwise adiabatic. It is thus evident thatthe wet-bulb thermometer 33 is essentially a means responsive tovariations in the total heat content of the air and hygroscopic liquidcirculatory systems and adapted to actuate means for increasing ordecreasing the total heat content of the two systems.

(2) As hereinbefore noted, if the drier is to" be operated on air ofconstant temperature and humidity, not only must the system remain atsubstantially constant total heat, but in addition the airreconditioning step in the chamber 1 must be carefully controlled so asto maintain constant conditions. That is, the air-hygroscopic liquidcontacting step must be operated so that the effectiveness thereof, i.e. the quantity of moisture removed from the total quantity of air aftereach passage through the chamber I, is such that the amount of waterabsorbed by the hygroscopic solution equals the amount of water givenoil by the'material being dried. This effectiveness depends upon therate of flow of air through the chamber 1, the rate of flow ofhygroscopic solution from the reservoir II into the sump I2, and theconcentration of the solution. In the apparatus illustrated, the rate ofair flow remains constant as long as the damper 8 is closed and the fan3 operates at constant speed; the concentration of the hygroscopicsolution is held constant by the density-responsive plummet 3I. Theeffectiveness of the reconditioning step is then dependent only on therate of flow of hygroscopic liquid. Accordingly, to provide automaticcontrol, the valve Iii governing this flow is connected operatively to adry-bulb thermometer or thermostat 32, placed in the drying chamber I,as shown, and set at the drying temperature desired. Then, when thetemperature of the air-reconditioning step in chamber 1.

Thus, if the valve I6 and density-device 29 be kept at a constantsetting, the thermostat 32 may be connected so as to vary the speed ofthe fan 3, or to control the quantity of air passing through the chamber1 by automatically controlling the relative positions of the dampers 6and 8. Again, if the valve l6, fan 3, and dampers 8 and 8 are at a'constant setting, the thermostat 32 may be connected to control theconcentration of the hygroscopic liquid, e. g. by varying the additionof heat to the evaporator 23 by means not illustrated, or by addingadditional quantities of one of the components to the solution. In theseinstances, the density control 29 and auxiliary parts are omitted. Inany of these modifications, the thermostat 32 may be considered as ameans responsive to variations in temperature in the drying chamber Iand adaptditioning operation in the chamber I.

As will be evident from the foregoing, the ultimate function of thethermostat 32 and the wetbulb thermometer 33 is to control thetemperature and humidity of the air entering the drier I. The wet-bulbthermometer 33 has hereinbelore been discussed as means responsive tothe total heat content of the system, i. e. wet-bulb temperature.However, as long as the dry-bulb temperature is under control, the totalheat content and humidity are interdependent, so that the wet-bulbthermometer 33 may equally well be considered as means responsive to thehumidity of the air. Thus, it has been found that any other humidityormoisture-responsive instrument, e. g. any hygrostat, will operateequally aswell as the wet-bulb thermometer 33. In-

deed, since in the apparatus the temperature and humidity are both underindependent control in an essentially adiabatic system, it has beenfound possible to interchange the connections of the thermostat 32 andhumidity device 33 without greatly affecting the operation of theapparatus. These interchanged controls are not, however; quite asrapidly responsive as controls connected .as illustrated, and huntingmay at times occur.

The process as particularly described above has been with reference,first, to adiabatic drying in the absence of any careful control oftemperature or humidity, and, then, as is preferable, with temperatureand humidity carefully maintained constant by suitable controls.However, with certain materials it is desirable to change gradually thestate of the air entering the chamber I during the course of the dryingoperation. This can be accomplished according to the present inventionby constantly or intermittently changing the setting of the thermostat32 and hygrostat 33 in any prescribed manner, the operation beingotherwise as described.

The process of the invention may be carried out using any hygroscopicliquid or solution, as sulfuric acid, glycerine, aqueous solutions ofcalcium and lithium halides, and the like. The process is not limited tothe removal of water by air drying, but is equally applicable to theremoval of any volatile liquid frommaterials moist therewith, using anyinert gas as the circulating medium, and any suitable absorbent liquidcapable of being reconcentrated.

It will be appreciated that in the process of the invention the dryingoperation in the chamber I, the air-reconditioning operation in thechamber I, and; the circulating system for the hygroscopic liquid allare maintained under sub-= stantially adiabatic conditions. It isentirely unnecessary to provide external means for heating the air goinginto the drying chamber or to discard any of the air leaving suchchamber, as is necessary in most drying methods hitherto known. The onlyheat requirement of the process, besides that for slight radiationlosses, if any, is the heat'required to reconcentrate the hygroscopicsolution. This quantity of heat my co-pending application Serial No,245,401,-

filed December 1'7, 1938, which, in turn is a continuation-impart of myprior application Serial No. 13,968, filed March 30, 1935. I

Other modes of applying the principle of the invention may be employedinstead of those explained, change being made as regards the detailsdisclosed, provided any of the steps or means stated in any of thefollowing claims or the equivalent of such stated steps or means beemployed.

I claim:

1. The method of removing a volatile liquid from a material wettherewith which comprises: circulating a body of an inert gas in aclosed system through a drying zone wherein it is contacted with the wetmaterial and removes part of the volatile liquid therefrom; as vapor,then at least in part through a reconditioning zone wherein it iscontacted with an absorbent liquor which removes part of the vapor ofthe volatile liquid and reconditions the gas for re-use, and finallyback to the drying zone; circulating the absorbent liquor employed inthe reconditioning zone into re-use in said zone; and withdrawingabsorbent liquor from the liquor circulatory system, reconcentrating thesame to an extent suf: ficient to maintain the concentration of theentire body of absorbent liquor substantially conthe gas and liquorcirculatory systems and the surrounding atmosphere and thus to maintainboth the said systems at substantially constant total heat content, andreturning the reconcentrated liquor to the liquor circulatory system;-

2. The method of removing a volatile liquid I from a material wettherewith which comprises:

circulating a body of an inert gas in a closed system through a dryingzone wherein it is contacted with the wet material and removes part ofthe volatile liquid therefrom as vapor, then at least in part through areconditioning zone wherein it is contactedwith an absorbent liquorwhich removes part 01' the vapor of the volatile liquid and reconditionsthe gas for re-use, and finally back to the drying zone; circulating theabsorbent liquor employed in the reconditioning zone through a closedsystem into re-use in said zone; and withdrawing a small portion of theabsorbent liquor from. the closed liquor circulatory system,reconcentrating the same to an extent suflicient to maintain theconcentration of the entire body of absorbent liquor substantiallyconstant, adjusting the temperature of the reconcentrated liquor to adegree sufllcient to compensate for small heat exchange effects betweenthe gas and liquor circulatory systems and the surrounding atmosphereand thus to maintain both the said systems at substantially constanttotal heat content, and returning the reconcentrated liquor to theliquorcirculatory system. FRANCIS R. BICHOWSKY.

